A gas turbine includes a compressor, a combustor, and a turbine. The compressor generates high-temperature and high-pressure compressed air, by compressing air taken in from an air inlet. The combustor generates high-temperature and high-pressure combustion gas by supplying fuel to the compressed air and burning it. The turbine includes a plurality of turbine vanes and turbine rotor blades alternatively arranged in a casing. For example, a rotor coupled to a generator is rotatably driven because the combustion gas supplied to an exhaust passage drives the turbine rotor blades. The combustion gas used to drive the turbine is converted into static pressure by a diffuser, and released to the atmosphere.
In such a gas turbine, the temperature of the combustion gas for driving the turbine is increased with high efficiency. Accordingly, almost all components of the turbine must be cooled, and the diffuser that is an exhaust unit of the turbine must also be cooled.
For example, Patent Document 1 discloses a conventional gas turbine for cooling a diffuser. This gas turbine includes an exhaust casing including an outer peripheral casing and an inner peripheral casing connected to the downstream side of a turbine. Struts are formed between the inner peripheral casing and the outer peripheral casing. Diffusers are provided at the outer peripheral side and the inner peripheral side of the exhaust casing, and an exhaust plenum including a strut cover for covering each of the struts and coupling the diffusers is provided. A first cooling system that supplies cooling air into the exhaust plenum, from the outer peripheral side of an outer peripheral diffuser to the inner peripheral side of an inner peripheral diffuser, through the inside of the strut cover, and a second cooling system that supplies cooling air along the inner peripheral diffuser at the further inner peripheral side of the inner peripheral diffuser are provided.
In this gas turbine disclosed in Patent Document 1, for example, as a gas turbine disclosed in Patent Document 2, the inner peripheral casing is supported by the outer peripheral casing by struts, and the struts are extendedly provided in the radial direction around the axial center of the rotor. A bearing part of the rotor is supported in the inner peripheral casing. In other words, in the gas turbines disclosed in Patent Document 1 and Patent Document 2, the rotor is supported by the inner peripheral casing, and the inner peripheral casing is supported by the outer peripheral casing by the struts. Accordingly, the outer peripheral casing, the inner peripheral casing, and the struts are formed as a rigid structure. Consequently, to maintain the position of the rotor in the gas turbine disclosed in Patent Document 1, the expansion of the outer peripheral casing, the inner peripheral casing, and the struts due to the heat of combustion gas must be prevented by cooling the outer peripheral casing, the inner peripheral casing, and the struts to a uniform temperature, with such cooling performed by the first cooling system and the second cooling system.
For example, in a gas turbine disclosed in Patent Document 3, a bearing case in a tubular shape for supporting the bearing part of the rotor is provided instead of the inner peripheral casing in Patent Document 1. A plurality of struts extending in the tangential direction of the bearing case is formed between the bearing case and the outer peripheral casing (casing wall), at equal intervals in the circumferential direction of the bearing case. In the gas turbine, the position of the rotor is maintained even if the struts are expanded or contracted by the changes in temperature because the bearing case rotates about the axial center of the rotor. Diffusers of the gas turbine disclosed in Patent Document 3 are provided at the inner side of the outer peripheral casing and at the outer peripheral side of the bearing case. The diffusers are coupled by a strut cover used to cover each of the struts, and a cooling chamber linked to the strut cover is formed at the outer peripheral side and the inner peripheral side of the diffusers.    [Patent Document 1] Japanese Patent Application Laid-open No. 2005-83199    [Patent Document 2] Japanese Patent Application Laid-open No. 2003-314299    [Patent Document 3] Japanese Patent Application Laid-open No. 2007-192028